The present invention relates to tonometry, the measurement of intraocular tension or pressure. More particularly, the present invention is concerned with a sensor means for use in tonometry for monitoring of intraocular pressure or tension.
Glaucoma is a disease in which the intraocular pressure is too high in a given eye. This elevated intraocular pressure produces a gradual loss of peripheral vision. Glaucoma is also characterized by hardening of the eyeball, and the disease, while leading to a gradual impairment of sight, can often result in blindness. There are a large number of people who are becoming visually impaired or going blind due to glaucoma, even with current diagnostic methods, modalities of treatment, the present level of knowledge and careful follow-up care.
It has been shown that some glaucoma victims had higher intraocular pressures than was originally thought, due to inaccuracy of current diagnostic methods which may not reliably evaluate the severity of a patient's condition. A current diagnostic method is to evaluate a patient's intraocular pressures on an intermittent basis, the intermittency being determined by a number of factors such as visual field defects, the health of the optic nerve, and the intraocular pressure levels. It has been shown from research testing that the intraocular pressure fluctuates widely and undergoes a diurnal variation. However, in general, most checking of a patient's intraocular pressure is done by the physician over a limited period of time between the hours of 8 a.m. to 10 p.m., which may not permit accurate monitoring of diurnal variations in the intraocular pressure.
Thus, for accurate assessment of a patient's intraocular pressure, the intraocular pressure level should be monitored over an extended period such as for 24 hours. Such extended monitoring, while highly desirable, is impractical due to the technical requirements of current testing methods and the fact that it would necessitate waking the patient every hour or every other hour as well as requiring the attendance of a physician. Consequently, intraocular pressure testing is generally not done during the period from 10 p.m. to 8 a.m., which period represents a significant portion of a patient's diurnal cycle. Furthermore, in the case that a patient's intraocular pressure is highest late at night or in the early morning hours, this would not be revealed by present testing methods.
The currently employed intraocular pressure testing devices and methods vary, but are based generally upon the well known principle that the pressure within a spherical body having a flexible surface, such as a balloon, can be measured by measuring the force required to deform the surface of the sphere. As applied to measuring intraocular pressure, this has been done in a number of ways.
The so-called "Shiotz tonometer" is a gravity indentation device which utilizes a blunt, weighted pin which rests against the eyeball. As the pin deforms the eye's surface, a linkage attached to the pin causes a pointer to move along a calibrated numerical scale, permitting the indicated intraocular pressure to be read from the scale.
The so-called "Applanation tonometer" is widely employed today and utilizes a small flat lens having a circle inscribed on its surface. This lens is placed against the surface of the eye and force is increasingly applied to deform the eye until the area of the eye surface thereby flattened against the flat lens matches the area of the inserted circle on the lens, whereupon the force is then read from a scale which is calibrated to the intraocular pressure.
In the so-called "air-test tonometer", a puff of air of a known volume and pressure is applied against the surface of the eye, while sensors detect the amount of deformation in the eye's surface caused by application of the puff of air. Such a device is described for example in U.S. Pat. No. 3,545,260.
The so-called "MacKay-Marg tonometer" is an applanation-type device in which a sensor is bounced off the cornea and the intraocular pressure is recorded graphically on a chart.
A common disadvantage of the currently used devices is that a physician and/or assistant is required to be in attendance during the testing procedure. In some cases, it is also necessary to medicate or topically anesthetize the patient's eye prior to testing. Thus, with the devices currently employed no practical means has been provided for measuring a patient's intraocular pressure around the clock and over an extended period of time, and without the attendance of a physician or technician. Consequently, with the currently available devices, the conventional practice is to routinely check a patient's intraocular pressure, for example at intervals of from two weeks to six months, and in such cases the intraocular pressure is only tested for a brief period of a few seconds at a time. Thus, the current testing practices are rather crude in that there is little assurance of an accurate reading being obtained during such a brief testing period, given the variations and fluctuations in intraocular pressures over a diurnal or longer period which cannot be practically monitored with the currently used testing devices and methods. Therefore, it may not be possible to reliably determine whether or not a patient's intraocular pressure is under control, nor to determine the proper treatment, nor to assess whether or not the patient may go blind.
Thus, there is presently a great need for a means for accurately monitoring a patient's intraocular pressure on an extended basis, such as over a period of 24 hours or longer. It is also desirable to monitor a patient's intraocular pressure under normal living conditions, and without the need for anesthesia or other drugs. It is further desirable to be able to monitor a patient's intraocular pressure without the constant attendance of a physician or technician. Still further, it is highly desirable to be able to monitor a patient's intraocular pressure outside the doctor's office or hospital and under various living conditions, such as while the patient is sleeping, exercising, relaxing, straining, at different altitudes, while under stress, etc.
The present invention is directed to meeting the above-mentioned needs as well as to overcoming the problems and disadvantages inherent in current devices and methods.
The intraocular pressure sensor of the present invention utilizes a small piezo-electric strain gauge for directly monitoring intraocular pressure. The piezo-electric strain gauge is mounted in a small holder which is shaped similar to a contact lens. The sensor is intended to be placed in contact with the sclerotic portion of the eyeball (sclera), that is, so that the sensor presses on the white part of the eye. The sensor is held in position against the eyeball by the eyelid as well as by the suction effect of the shaped holder. By making the sensor small, it is possible to place the sensor in the lower cul-de-sac of the eye behind the lower eyelid. Wire leads from the strain gauge are passed through the holder and along and out over the lower eyelid, and these wire leads may be connected to a suitable power source and monitoring/recording circuitry for receiving the sensor output signal and converting the output signal into a pressure value.
It is known to use piezo-electric and other devices for remotely monitoring bodily pressures. For example, U.S. Pat. No. 3,239,696 discloses a piezo-electric pressure transducer for measuring cardiovascular circulatory system pressure in the human body. U.S. Pat. No. 4,023,562 discloses a miniature piezo-resistive transducer device adapted to be implanted within the body for directly monitoring internal fluid or pneumatic pressures therein, utilizing semiconductor strain gauge elements including a piezo-resistive bridge.
U.S. Pat. No. 3,948,248 describes placing a piezo-resistive weight on the corneal surface to record intraocular pressures, in connection with the ultrasonic detection of intraocular pressures. In U.S. Pat. No. 3,903,871 there is disclosed a portable compression-type opthalmodynamometer for determining retinal artery pressures, while in U.S. Pat. No. 4,281,662 there is disclosed a microprocessor-controlled device which stores the magnitude value data of systolic and diastolic pressures and calculates the percentage difference therebetween. Also disclosed are a display and bar graph as well as a strip chart recorder.
However, the known devices fail to offer a suitable sensor for monitoring intraocular pressure which can provide accurate intraocular pressure readings over an extended period while worn in the eye under various living conditions.
It is therefore an object of the present invention to provide an intraocular pressure sensor which enables the intraocular pressure to be accurately monitored on an extended basis over a long period of time.
It is also an object of the present invention to provide an intraocular pressure sensor which permits the intraocular pressure to be monitored under normal living conditions.
It is additionally an object of the present invention to provide an intraocular pressure sensor which permits intraocular pressures to be monitored without a doctor or technician in attendance.
It is further an object of the present invention to provide an intraocular pressure sensor which can be worn in the eye like a contact lens.
It is still further an object of the present invention to provide an intraocular pressure sensor which permits a patient's intraocular pressure to be tested without the necessity for administering medication or anethesia.